CN113646459A

CN113646459A - Method for producing black-cored malleable cast iron member formed by plating, and black-cored malleable cast iron member formed by plating

Info

Publication number: CN113646459A
Application number: CN202080020701.5A
Authority: CN
Inventors: 后藤亮; 深谷刚千; 松井博史; 泽田明典
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2019-03-20
Filing date: 2020-03-16
Publication date: 2021-11-12
Anticipated expiration: 2040-03-16
Also published as: CN113646459B; WO2020189637A1; JPWO2020189637A1; JP7375809B2

Abstract

The present invention provides a method for manufacturing a black-cored malleable cast iron member formed by plating, the black-cored malleable cast iron member formed by plating having a plating layer formed on a surface of the black-cored malleable cast iron member, the method comprising: graphitizing the graphite body in a non-oxidizing and decarburizing atmosphere; performing a particle projection treatment on the surface of the graphitized black-cored malleable cast iron member so that silicon oxide remains on the surface; immersing the black-cored malleable cast iron member subjected to the particle projection treatment in a flux for 3.0 minutes or more; and a step of hot dip coating the flux-dipped black heart malleable cast iron member.

Description

Method for producing black-cored malleable cast iron member formed by plating, and black-cored malleable cast iron member formed by plating

Technical Field

The present invention relates to a method for producing a plated black-cored malleable cast iron member, and a plated black-cored malleable cast iron member produced by the production method, and particularly to a pipe joint.

Background

Cast irons are classified into flake graphite cast irons, spheroidal graphite cast irons, malleable cast irons, and the like according to the presence of carbon. The malleable cast iron is further classified into white-cored malleable cast iron, black-cored malleable cast iron, and pearlitic malleable cast iron, etc. Black-cored malleable iron, which is the object of the present invention, is also called malleable iron, and has a mode in which graphite is dispersed in a matrix composed of ferrite. In the process of producing black-cored malleable cast iron, carbon after casting and cooling is present as cementite, which is a compound with iron. Then, the casting is heated and maintained at a temperature of 720 ℃ or higher, whereby the cementite is decomposed to precipitate graphite. In the present specification, the step of precipitating graphite by heat treatment will be hereinafter referred to as "graphitization".

Black-cored malleable iron has excellent mechanical strength as compared with flake graphite cast iron, and also has excellent toughness because its matrix is ferrite. Therefore, black-cored malleable cast iron is widely used as a material for constituting parts such as automobile parts and pipe joints which require mechanical strength. Hot dip galvanizing for corrosion prevention is often performed on the surface of a pipe joint formed of black malleable cast iron. The hot-dip galvanized layer is excellent in durability and can be plated at a low cost, and therefore, is suitable as a corrosion prevention method for pipe joints.

Conventionally, oxides of iron, silicon, and the like are easily generated in the process of graphitization on the surface of a member made of black-cored wrought cast iron (hereinafter referred to as "black-cored wrought cast iron member"). When the plating layer is formed on the surface on which these oxides are formed, the surface of the base material is likely to be exposed to the outside without a plating film (hereinafter, may be referred to as "no plating"). Therefore, in order to form a plating layer having good adhesion on the black-cored malleable cast iron member, it is necessary to prepare a black-cored malleable cast iron member having a surface in which the generation of oxides is suppressed as much as possible, and form a plating layer on the surface thereof.

For the purpose of producing a black-cored malleable cast iron member having a small amount of oxide on the surface, various methods for removing oxide formed on the surface have been studied. For example, patent document 1 describes a method of removing oxides by immersing a black-cored malleable cast iron member in an acidic solution. This process is sometimes referred to as "pickling". For example, patent document 2 describes a method of removing oxides formed on the surface of a black heart malleable cast iron member by shot blasting for a long time.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-19878

Patent document 2: japanese laid-open patent publication No. 58-151463

Patent document 3: international publication No. 2013/146520

Disclosure of Invention

(problems to be solved by the invention)

The pickling described in patent document 1 has problems that the acidic solution itself, gases generated by the reaction with the black heart malleable cast iron, and the like are harmful to the human body and require attention in handling, that the acidic solution after use is discarded, that the generated gases are discharged to the outside, and that the load on the environment is large. In addition, the method disclosed in patent document 2 has a problem that it is difficult to form a thermal dip coating layer on the surface satisfactorily.

The present invention has been made in view of the above problems, and an object thereof is to produce a black-cored malleable cast iron member having a hot-dip plated layer formed on a surface thereof in a satisfactory manner without pickling.

(means for solving the problems)

Mode 1 of the present invention is a method for producing a black-cored malleable cast iron member formed by plating,

the black-cored malleable cast iron member formed by plating has a plating layer formed on the surface of the black-cored malleable cast iron member, and the manufacturing method has the following steps:

graphitizing the graphite body in a non-oxidizing and decarburizing atmosphere;

performing a particle projection treatment on the surface of the graphitized black-cored malleable cast iron member so that silicon oxide remains on the surface;

immersing the black-cored malleable cast iron member subjected to the particle projection treatment in a flux for 3.0 minutes or more; and

and a step of hot dip coating the flux-dipped black heart malleable cast iron member.

Embodiment 2 of the present invention is the method for producing a black heart malleable cast iron member formed by plating according to embodiment 1, wherein the non-oxidizing and decarburizing atmosphere is an atmosphere having an oxygen partial pressure 10 times or less higher than an equilibrium oxygen partial pressure of the following chemical formula (1) and higher than an equilibrium oxygen partial pressure of the following chemical formula (2),

[ chemical formula 1]

2Fe(s)+O₂(g)＝2FeO(s) (1)

[ chemical formula 2]

2C(s)+O₂(g)＝2CO(g) (2)。

Embodiment 3 of the present invention is the method for manufacturing a black heart malleable cast iron member formed by plating according to embodiment 1 or 2, wherein the particle projection treatment is any one of shot blast (shot blast), shot peening (shot peen), sand blast (sand blast), and air blast (air blast).

Embodiment 4 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 3, wherein the particle projection treatment is performed for 3.0 minutes to 20 minutes.

Embodiment 5 of the present invention is the method for producing a black heart malleable cast iron member formed by plating according to any one of embodiments 1 to 4, further including a step of preheating the black heart malleable cast iron member at a temperature of 275 ℃ or higher and 425 ℃ or lower before the step of graphitizing.

Embodiment 6 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 5, wherein the step of performing graphitization includes a1 st graphitization in which heating is performed at a temperature exceeding 900 ℃, and a 2 nd graphitization in which a start temperature is 720 ℃ or more and 800 ℃ or less, and an end temperature is 680 ℃ or more and 780 ℃ or less.

Embodiment 7 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to embodiment 6, wherein in the step of graphitizing, at least the 1 st graphitization is performed in a non-oxidizing and decarburizing atmosphere.

Embodiment 8 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 7, wherein the non-oxidizing and decarburizing atmosphere includes a reformed gas generated by combustion of a mixed gas of a combustion gas and air.

Embodiment 9 of the present invention is the method for manufacturing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 8, further including: and a step of heating the black-cored malleable cast iron member to 90 ℃ or higher after taking out the member from the flux.

Embodiment 10 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 9, wherein the flux is an aqueous solution containing a weakly acidic chloride.

Embodiment 11 of the present invention is the method for producing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 10, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

Embodiment 12 of the present invention is the method for producing a plated black-cored malleable cast iron member according to any one of embodiments 1 to 11, wherein the step of performing hot dip plating includes a step of performing hot dip galvanizing.

Embodiment 13 of the present invention is the method for manufacturing a black-cored malleable cast iron member formed by plating according to any one of embodiments 1 to 12, wherein the black-cored malleable cast iron member is a pipe joint.

Embodiment 14 of the present invention is a black-cored malleable cast iron member formed by plating, characterized in that,

the black-cored malleable cast iron member formed by plating has a plating layer formed on the surface of the black-cored malleable cast iron member,

the coating layer is a hot dip galvanizing layer,

the cast iron surface of the black-cored malleable cast iron member has a work-modified region, and

the hot-dip galvanized layer includes silicon oxide.

Embodiment 15 of the present invention is the black-cored malleable cast iron member formed by plating according to embodiment 14, wherein the black-cored malleable cast iron member formed by plating is a pipe joint.

(effect of the invention)

According to the method for producing a black-cored malleable cast iron member formed by plating according to the embodiment of the present invention, the surface adjustment suitable for the generation of the plating layer can be performed by the graphitization step which is indispensable in the production, and the pickling step which is indispensable in the formation of the conventional plating layer can be omitted. In addition, by the flux treatment which is specified as the light particle projection treatment, the plating failure can be reliably prevented. As a result, the production of the black-cored malleable cast iron member having the plated layer can reduce the load on the environment and can reduce the cost as compared with the conventional one.

Drawings

Fig. 1A is an example of a reflection electron image of a cross section near the surface of a black heart malleable cast iron member after graphitization and before particle projection treatment in the example.

Fig. 1B is an elemental mapping image of silicon in the same area as fig. 1A.

Fig. 1C is an elemental mapping image of oxygen in the same region as fig. 1A.

Fig. 2 is an example of a reflection electron image of the surface of the black heart malleable cast iron member after graphitization and before the particle projection treatment in the example.

Fig. 3 is an example of a reflected electron image of a cross section near the surface of a black heart malleable cast iron member before flux dipping after shot blasting as a particle projection process in the example.

Fig. 4 is an example of a reflection electron image of the surface of a black heart malleable cast iron member before flux dipping after shot blasting as a particle projection process in the example.

Fig. 5 is an example of a reflection electron image of a cross section including the total thickness of the plating layers of the black heart malleable cast iron part after hot dip plating in the example.

Fig. 6 is an example showing a reflection electron image in the vicinity of the boundary of the cast iron surface and the plating layer of the black heart malleable cast iron member after hot dip plating in the embodiment.

Detailed Description

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings and tables. The embodiments described herein are merely examples, and the embodiments for carrying out the present invention are not limited to the embodiments described herein. The mechanism described here is merely an assumption that the present inventors can reasonably explain the fact that the present inventors have known, and does not limit the technical scope of the present invention.

In the production process of the black-cored malleable cast iron member formed by plating, when the plating is performed on the black-cored malleable cast iron member such as a pipe joint without pickling, the quality of the plating depends on the state of the surface of the black-cored malleable cast iron member. In addition, the shape of the black-cored malleable cast iron member and the plating conditions also affect the quality of the plating. For example, in the case of manufacturing a pipe joint as a cast iron member, when the immersion time in the plating bath is short, minute non-plating having a diameter of several mm or less tends to occur easily on the inner surface of the pipe joint. Even if the pipe joint in which the minute unplated pipe joints are generated is again immersed in the plating bath, there are the following problems: the plating layer is not formed on the non-plated portion, and it is difficult to repair the non-plating.

Further, the present inventors have focused on the following points: the quality of plating also depends on the state of immersion of the black heart malleable cast iron member when the black heart malleable cast iron member is immersed in the plating bath, and more specifically, the poor plating also depends on the rising of the black heart malleable cast iron member such as a pipe joint above the plating bath when the pipe joint is immersed in the plating bath (hereinafter, this phenomenon may be referred to as "rising" (japanese abbreviated as "pot rising き")). If the above-mentioned floating occurs, there are the following problems: the thickness of the plating layer becomes uneven, or pinholes due to bubbles are formed in the plating layer.

In particular, when the dipping time in flux is short or when the weight of the black-cored malleable cast iron member is light, the above-described floating tends to occur easily. Further, the shape of the pipe joint tends to be complicated as in the later-described embodiments. As one of the specific reasons for the occurrence of the floating, the following is considered: after the black-cored malleable cast iron member is immersed in the plating bath, the flux adhering to the surface of the black-cored malleable cast iron member is rapidly heated in the plating bath, whereby a certain chemical reaction occurs, and a gas is generated on the surface of the black-cored malleable cast iron member, and this gas is retained in the pipe joint in the form of bubbles. However, the floating may occur even when the air bubbles are not generated or when the air bubbles are discharged to the outside of the pipe joint.

In order to solve the above-described problems, the present inventors have made extensive studies on a method for producing a plated black-cored malleable cast iron member, in order to obtain a plated black-cored malleable cast iron member which is suppressed in particular from floating irrespective of bubbling and is suppressed from plating failure such as no plating without carrying out pickling as in the prior art. As a result, it was found that a black heart malleable cast iron member having a surface suitable for forming a plating layer can be obtained without performing an acid washing treatment by performing graphitization, which is indispensable for the production of a black heart malleable cast iron member formed by plating, in a specific atmosphere, performing a light particle projection treatment, and performing immersion in a flux under specific immersion conditions, and that the plating layer can be formed well at the time of plating formation while sufficiently suppressing floating at the time of immersion in a plating bath. Details will be described below.

In the present specification, the black-cored malleable cast iron member on which the plating layer is formed is referred to as a "black-cored malleable cast iron member formed by plating". Further, the cast iron portion in contact with the plating layer of the black-cored malleable cast iron member formed by plating is sometimes particularly referred to as "cast iron surface".

< composition of alloy >

The main material constituting the black heart malleable cast iron member in the present invention is black heart malleable cast iron. The black-cored malleable cast iron preferably contains 2.0 to 3.4 mass% of carbon, 0.5 to 2.0 mass% of silicon, and iron and inevitable impurities as the remainder. When the carbon content is 2.0 mass% or more, the fluidity of the molten metal is good, so that the casting operation becomes easy and the fraction defective due to the molten metal flow of the molten metal can be reduced. If the carbon content is 3.4 mass% or less, graphite precipitation during casting and during cooling after casting can be prevented. If the content of silicon is 0.5 mass% or more, the effect of promoting graphitization by silicon can be obtained, and graphitization can be completed in a short time. If the silicon content is 2.0 mass% or less, graphite precipitation during casting and during cooling after casting can be prevented.

The black heart malleable cast iron in the embodiment of the present invention further preferably further contains 1 or 2 elements selected from the group of elements consisting of bismuth and aluminum in a total amount of 0.005 mass% or more and 0.020 mass% or less. If the total content of bismuth and aluminum is 0.005 mass% or more, graphite precipitation during casting and during cooling after casting can be prevented. If the total content of bismuth and aluminum is 0.020% by mass or less, graphitization is not greatly inhibited. In addition to the above elements, the black heart malleable cast iron in the embodiment of the present invention may contain 0.5 mass% or less of manganese.

< preheating >

In a preferred embodiment of the present invention, the black heart malleable cast iron member before graphitization is preferably preheated at a temperature of 275 ℃ or more and 425 ℃ or less. In the present invention, "preheating" refers to heat treatment in a low temperature region performed on a black cast iron member after casting, before graphitization. By performing the preheating, the graphite after graphitization is dispersed and present at the positions of the grain boundaries of the ferrite, and the grain size of the ferrite can be made finer than that of the conventional black-cored wrought cast iron. In addition, the time required for graphitization can be shortened. The above-described effect of preheating is more pronounced when the black heart malleable cast iron member contains 1 or 2 elements selected from the group of elements consisting of bismuth and aluminum.

The present invention relates to the formation of a plating layer, and therefore, the alloy composition of a black-cored malleable cast iron member in the method for producing a black-cored malleable cast iron member formed by plating according to the present invention is not limited to a specific alloy composition. The alloy composition in the present invention may be any alloy composition as long as it does not significantly deviate from the range of the alloy composition described above which is generally considered as an alloy composition of black-cored malleable cast iron. Similarly, the crystal grain size of the ferrite after graphitization is not particularly limited in the present invention. Therefore, the preheating is not an essential step in the present invention, and it is needless to say that the plating layer is allowed to be formed on the surface of the black-cored malleable cast iron that is graphitized without preheating in the present invention.

< temperature and holding time for graphitization >

In the method for producing a black-cored malleable cast iron member formed by plating according to the present invention, the black-cored malleable cast iron member after casting, preferably the previously preheated black-cored malleable cast iron member, is heated and held at a temperature of, for example, 720 ℃ or higher, and is preferably subjected to a heat treatment called graphitization. Graphitization is a process inherent in the manufacturing process of black-cored malleable cast iron. In the graphitization step, toughness can be imparted to the black-cored wrought cast iron member by heating the black-cored wrought cast iron member to a temperature exceeding, for example, 720 ℃ corresponding to the a1 transformation point, thereby decomposing cementite to precipitate graphite, and cooling the matrix formed of austenite to transform it into ferrite. The graphitization is preferably classified into 1 st graphitization performed initially and 2 nd graphitization performed after the 1 st graphitization.

The 1 st graphitization is preferably a step of decomposing cementite in austenite in a temperature range exceeding 900 ℃ to precipitate graphite. In the 1 st graphitization, carbon separated by decomposition of cementite contributes to the production of graphite. The temperature for carrying out the 1 st graphitization is more preferably 920 ℃ or more and 980 ℃ or less. The holding time required for the 1 st graphitization varies depending on the size of the black heart malleable cast iron part to be graphitized. In the case of performing the preheating, the holding time for the 1 st graphitization is preferably 30 minutes or more and 3 hours or less, and more preferably 2 hours or less.

The 2 nd graphitization is preferably a step of transforming austenite to ferrite and decomposing cementite in ferrite and/or pearlite in a temperature region lower than the temperature at which the 1 st graphitization is performed to precipitate graphite. The 2 nd graphitization is preferably performed while gradually decreasing the temperature from the 2 nd graphitization start temperature to the 2 nd graphitization end temperature. This makes it possible to precipitate graphite while gradually decreasing the solid solubility of carbon in austenite, and thus the transformation from austenite to ferrite is reliably performed.

The graphitization start temperature 2 is preferably 720 ℃ to 800 ℃. The 2 nd graphitization termination temperature is preferably 680 ℃ or more and 780 ℃ or less, more preferably 720 ℃ or less, and is a temperature lower than the 2 nd graphitization initiation temperature. The time required from the start to the completion of the 2 nd graphitization also differs depending on the size of the black heart malleable cast iron member to be graphitized. In the case of performing the preheating, the time for graphitizing the 2 nd graphite is preferably 30 minutes to 3 hours, and more preferably 2 hours. In the transition from the 1 st graphitization to the 2 nd graphitization, it is preferable to lower the temperature from the 1 st graphitization temperature to the 2 nd graphitization start temperature. In the embodiment of the present invention, the following manner cannot be performed: the temperature is decreased from the 1 st graphitization temperature to a temperature lower than the 2 nd graphitization start temperature, for example, to room temperature, and then increased to the 2 nd graphitization start temperature. The time required for the temperature reduction at the time of the transition from the 1 st graphitization to the 2 nd graphitization is not particularly limited.

< non-oxidizing atmosphere >

In the method for producing a black-cored malleable cast iron member formed by plating according to an embodiment of the present invention, graphitization of the black-cored malleable cast iron member is performed in a non-oxidizing and decarburizing atmosphere. The "non-oxidizing atmosphere" in the present invention does not mean only a strictly reducing atmosphere, i.e., an atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of chemical formula 1 described later at the graphitization temperature, but means an atmosphere in which iron contained in the black heart malleable cast iron member reacts with a gas constituting the atmosphere to form iron oxides to such an extent that the formation of the plating layer is not hindered. That is, the "non-oxidizing atmosphere" in the present invention is a broad concept including an atmosphere in which a thick oxide layer is not formed to such an extent that the formation of the plating layer is not hindered. Specifically, the "non-oxidizing atmosphere" is preferably an atmosphere in which the partial pressure of oxygen in the graphitization atmosphere is 10 times or less the equilibrium partial pressure of oxygen of chemical formula 1 described in detail below. Therefore, according to a preferred embodiment, the equilibrium oxygen partial pressure of chemical formula 1 at the temperature at which graphitization is performed is determined, and the non-oxidizing atmosphere in the present invention is included in cases where graphitization is performed in a state equal to or lower than the determined equilibrium oxygen partial pressure, except for cases where the oxygen partial pressure in the atmosphere for graphitization is 10 times or less the above-described equilibrium oxygen partial pressure of chemical formula 1. The oxygen partial pressure in the atmosphere for graphitization is more preferably 6 times or less, even more preferably 3 times or less, and even more preferably equal to or less than the equilibrium oxygen partial pressure of chemical formula 1.

A chemical formula representing a representative reaction in the oxidation reaction of iron is shown in chemical formula 1.

[ chemical formula 3]

2Fe(s)+O₂(g)＝2FeO(s) (1)

Herein, fe(s) represents solid iron, O2(g) represents gaseous oxygen, and feo(s) represents solid ferrous oxide (Wustite). Several reactions are known for the oxidation reaction of iron in addition to chemical formula 1, but the oxidation reaction at the lowest temperature of graphitization standard gibbs energy is the reaction of chemical formula 1. Therefore, in an atmosphere in which the oxidation reaction of iron represented by chemical formula 1 is difficult to progress, the oxidation reaction of iron represented by other chemical formulas is also difficult to progress.

When graphitization is performed in a non-oxidizing atmosphere, the equilibrium oxygen partial pressure of chemical formula 1 at the temperature at which graphitization is performed is determined, and the oxygen partial pressure of the atmosphere is preferably 10 times or less the equilibrium oxygen partial pressure of chemical formula 1, as described above. It is particularly preferable that the oxygen partial pressure of the atmosphere is equal to or lower than the determined equilibrium oxygen partial pressure. In this way, the reaction of chemical formula 1 is maintained in chemical equilibrium or proceeds from right to left, and further generation of iron oxide is sufficiently inhibited. The value of the equilibrium oxygen partial pressure of chemical formula 1 at the graphitization temperature can be calculated using the literature value of the standard gibbs energy of chemical formula 1 (non-patent document 1). Table 1 shows an example of calculating the equilibrium oxygen partial pressure of chemical formula 1 for the 1 st graphitization (980 c) and the 2 nd graphitization (760 c). In performing this calculation, reference is made to the standard gibbs energy values reported in m.w. chase, "NIST-JANAF thermal Tables", (usa), 4 th edition, American Institute of Physics, 8/1/1998.

[ Table 1]

In order to know whether or not the oxygen partial pressure of the atmosphere in graphitization is equal to or lower than the equilibrium oxygen partial pressure of chemical formula 1 shown in table 1 and whether or not the oxygen partial pressure of the atmosphere in graphitization is several times the equilibrium oxygen partial pressure of chemical formula 1, it is necessary to know the oxygen partial pressure of the atmosphere. Examples of the method for measuring the oxygen partial pressure of the atmosphere include a method for directly measuring the oxygen partial pressure of the atmosphere using a zirconia oxygen concentration meter, a quadrupole mass spectrometer, or the like. However, when the extremely low oxygen partial pressure shown in table 1 is measured, the measurement accuracy may not necessarily be sufficient in the above-described direct method.

When a reformed gas is used as the graphitization atmosphere gas, for example, as described in patent document 3, the partial pressure ratio of carbon monoxide to carbon dioxide or the partial pressure ratio of hydrogen to water vapor in the atmosphere is measured, and the partial pressure of oxygen in equilibrium with the above gas is indirectly obtained by calculation. This calculation is considered to be a reaction (2CO + O) in which carbon monoxide reacts with oxygen to generate carbon dioxide in the heat treatment furnace₂＝2CO₂) Or reaction of hydrogen with oxygen to form water vapor (2H)₂+O₂＝2H₂O) is established.

In the embodiment of the present invention, as the method of making the atmosphere for graphitization a non-oxidizing atmosphere, a known method capable of reducing the partial pressure of oxygen can be used. Specific examples of the method include, but are not limited to, a method of maintaining a high vacuum in a heat treatment furnace, and a method of filling the heat treatment furnace with a non-oxidizing gas.

In a preferred embodiment of the present invention, the non-oxidizing atmosphere comprises converted gas (converted gas) produced by combustion of a mixed gas of a combustion gas and air. Since the converted gas can be produced relatively inexpensively, the production cost required for graphitization can be suppressed as compared with the case of using other non-oxidizing atmospheres. As the combustion gas that can be used for the generation of the reformed gas, there are propane gas, butane gas, a mixed gas thereof, liquefied petroleum gas, liquefied natural gas, and the like.

In the generation of the converted gas, a gas generating device may be used. CO is produced if the mixing ratio of air mixed with combustion gas is increased₂Gas and N₂And a complete combustion type gas having a large gas component. If the mixing ratio of air is reduced, CO gas and H gas are generated₂And an incomplete combustion type gas having a large gas component. The water vapor contained in the conversion gas can be partially removed by the freeze-dryer.

In the case of using a conversion gas in the formation of a non-oxidizing atmosphere, the conversion gas can be obtained by any of the methods described aboveIt is known that when the oxygen partial pressure in the heat treatment furnace is much higher than the equilibrium oxygen partial pressure of chemical formula 1 shown in Table 1, the mixing ratio of air mixed with combustion gas can be decreased to increase the ratio of CO gas to H gas₂The oxygen partial pressure is reduced by either reducing the ratio of the gas or the cooling temperature of the freeze-dryer to lower the dew point of the converted partial pressure. Alternatively, both of the above methods may be used.

In the embodiment of the present invention, as will be described later, graphitization is performed in the above-described non-oxidizing and decarburizing atmosphere, that is, graphitization is also performed in a decarburizing atmosphere, but it is not so important to use a graphitization atmosphere as a non-oxidizing atmosphere as compared with a decarburizing atmosphere. That is, even if a slight oxide layer is formed on the surface of the black-cored malleable cast iron member during graphitization, the formation of the plating layer may not be a significant obstacle. Therefore, the "non-oxidizing atmosphere" in the present invention is a broad concept as described above.

In a preferred embodiment of the present invention, the 2 nd graphitization may be performed under a reducing atmosphere, i.e., an atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of the above chemical formula 1. Even in the case where an oxide is generated on the surface of the black-cored malleable cast iron member in the 1 st graphitization, the temporarily generated oxide is reduced by performing the 2 nd graphitization in a reducing atmosphere, and the thickness of the oxide is reduced to such a thickness that the formation of the plating layer is not hindered.

< decarbonizing atmosphere >

In the method for producing a black-cored malleable cast iron member formed by plating according to the present invention, the atmosphere for graphitization of the black-cored malleable cast iron member is also a decarburization atmosphere. In the present invention, the "decarburizing atmosphere" refers to an atmosphere in which carbon contained in the black-cored malleable cast iron member is oxidized by oxygen in the atmosphere to become carbon monoxide, and carbon monoxide gas escapes from the surface of the black-cored malleable cast iron member to the outside to remove carbon. This chemical reaction can be represented by the following chemical formula 2.

[ chemical formula 4]

2C(s)+O₂(g)＝2CO(g) (2)

In this case, the amount of the solvent to be used,c(s) represents solid carbon, O₂(g) Oxygen, which represents gas, and CO (g) carbon monoxide, which represents gas. The oxidation reaction of carbon includes a reaction (C + O) in which carbon reacts with oxygen to generate carbon dioxide, in addition to chemical formula 2₂＝CO₂) However, in the temperature range of 720 ℃ or higher in which graphitization is performed, the reaction of chemical formula 2 having a low standard Gibbs energy preferentially proceeds.

When graphitization is performed in a decarburizing atmosphere, the equilibrium oxygen partial pressure of chemical formula 2 at the temperature at which graphitization is performed is determined, and graphitization may be performed in a state where the oxygen partial pressure of the graphitization atmosphere is higher than the equilibrium oxygen partial pressure. In this way, the reaction of chemical formula 2 proceeds from left to right, and carbon contained in the black-cored malleable cast iron reacts with oxygen to be carbon monoxide, which is released to the outside, and decarburization is performed. The value of the equilibrium oxygen partial pressure of chemical formula 2 at the graphitization temperature can be calculated using the literature value of the standard Gibbs energy, as in the case of chemical formula 1 described above. Table 1 also shows an example of calculating the equilibrium oxygen partial pressure of chemical formula 2 in the 1 st graphitization (980 ℃ C.) and the 2 nd graphitization (760 ℃ C.).

In order to know whether or not the oxygen partial pressure of the atmosphere for graphitization is higher than the equilibrium oxygen partial pressure of chemical formula 2 shown in table 1, it is necessary to measure the oxygen partial pressure of the atmosphere. The method of measuring the oxygen concentration of the atmosphere has already been described, and therefore, the description thereof is omitted here. When the oxygen partial pressure of the atmosphere obtained is higher than the equilibrium oxygen partial pressure of chemical formula 2 shown in table 1, graphitization can be performed in the state of the decarburization atmosphere. In the case where the converted gas is used as the atmosphere, when the oxygen partial pressure in the heat treatment furnace is equal to or lower than the equilibrium oxygen partial pressure of chemical formula 2, for example, the oxygen partial pressure may be adjusted to be higher than the equilibrium oxygen partial pressure of chemical formula 2 by increasing the air mixing ratio in the converted gas generation apparatus or by increasing the dew point of the converted gas. However, the method of adjusting the oxygen partial pressure is not limited thereto.

In the embodiment of the present invention, graphitization is performed in a decarburizing atmosphere, and thus graphite is not generated on the surface of the black heart malleable cast iron member during graphitization. Therefore, according to the production method of the present invention, a black-cored malleable cast iron member in which graphite is hardly generated on the surface before the plating layer is formed after graphitization can be produced, and a plating layer having excellent adhesion can be formed on the surface.

In the embodiment of the present invention, the graphitization of both the 1 st graphitization and the 2 nd graphitization may be performed in a non-oxidizing and decarburizing atmosphere, but if not, it is also preferable that at least the 1 st graphitization is performed in a non-oxidizing and decarburizing atmosphere. In the latter case, it is considered that the 2 nd graphitization is performed in an atmosphere of a non-decarburizing atmosphere. However, the 2 nd graphitization temperature is lower than the 1 st graphitization, so that the rate of graphite precipitation on the surface of the black heart malleable cast iron member is slower than that of the 1 st graphitization. Therefore, by carrying out at least the 1 st graphitization in a decarburizing atmosphere, the effects of the present invention can be obtained.

As described above, the method for producing a black-cored malleable cast iron member formed by plating according to the present invention includes a step of graphitizing in a non-oxidizing and decarburizing atmosphere. For example, when the 1 st graphitization (980 ℃ C.) is performed to produce a non-oxidizing and decarburizing atmosphere, an example is to make the oxygen partial pressure in the furnace higher than the equilibrium oxygen partial pressure of chemical formula 2 shown in Table 1, that is, 2.6X 10^-19atm, and 3.4X 10 which is the equilibrium oxygen partial pressure of chemical formula 1 shown in Table 1^-16atm or less.

As described above, according to the method for producing a black-cored malleable cast iron member formed by plating according to the present invention, a surface suitable for the formation of a plating layer can be prepared by utilizing a graphitization step which is indispensable for the production. In particular, since graphitization is performed in a decarburizing atmosphere, graphite, which is one of the causative substances of no plating, is hardly formed on the surface of the black-cored malleable cast iron member. Further, since graphitization is performed in a non-oxidizing atmosphere, there is almost no oxide layer and it is very thin if any. Thus, a surface of the black heart malleable cast iron member suitable for plating formation can be obtained.

< ferrite layer >

In a preferred embodiment of the present invention, the black heart malleable cast iron part after graphitization, before the particle projection treatment described below, has a ferrite layer on its surface with a thickness exceeding 100 μm. The ferrite layer is a layered structure composed of ferrite containing almost no carbon called an α phase in an iron-carbon two-membered diagram. In a preferred embodiment, as a result of decarburization of the surface of the black-cored malleable cast iron member, austenite having a small amount of carbon is produced, and if cooling is performed after graphitization is completed, a ferrite layer having a thickness of more than 100 μm is formed. If a ferrite layer is formed, graphite does not exist not only on the surface of the black-cored wrought cast iron member but also in the vicinity of the surface layer. Therefore, a plating layer having a stronger strength and excellent adhesion can be formed, which is preferable.

White heart malleable cast iron may be decarburized in a decarburizing atmosphere, but black heart malleable cast iron and pearlitic malleable cast iron are not generally graphitized in a decarburizing atmosphere. However, in the present invention, graphitization is performed in a decarburizing atmosphere for the purpose of forming a plating layer having excellent adhesion. Thus, even if a ferrite layer is formed on the surface of the black-cored malleable cast iron member, the influence on the mechanical properties is small as long as the thickness of the ferrite layer is not so large.

In the embodiment of the present invention, when a ferrite layer is formed on the surface of the black-cored malleable cast iron member, an oxide layer having a thin iron may be formed on the surface of the ferrite layer. Even if the oxide layer is formed, it can be removed by the particle projection treatment and the flux treatment which are the subsequent steps as long as the oxide layer is thin. Further, the formation of a thin oxide layer is preferable because it prevents excessive decarburization of the surface of the black-cored malleable cast iron member. The allowable thickness of the oxide layer that can be formed on the surface of the ferrite layer is preferably 20 μm or less, and more preferably 10 μm or less.

< silicon oxide >

In the embodiment of the present invention, silicon oxide is present on the surface of the black heart malleable cast iron member after the graphitization is completed. As shown above, silicon is one of the elements constituting the black heart malleable cast iron. Silicon is an element that is more easily oxidized than iron and carbon. Therefore, even when graphitization is performed in a non-oxidizing atmosphere in the present invention, it is inevitable that silicon contained in the black heart malleable cast iron is oxidized to generate silicon oxide. Silicon oxides generated during graphitization are mainly present on the surface of the black heart malleable cast iron parts. When the ferrite layer is formed on the surface of the black-cored malleable cast iron member, silicon oxide exists on the surface of the ferrite layer. As described above, if oxides are formed on the surface of the black-cored malleable cast iron member, the oxides cause no plating. However, in the present invention, by performing the particle projection treatment described later on the silicon oxide present on the surface of the black-cored malleable cast iron member, non-plating can be prevented.

< particle projection processing >

In an embodiment of the present invention, the surface of the black-cored malleable cast iron member is subjected to a particle projection treatment so that silicon oxide remains on the surface before being dipped in flux after the graphitization. The particle projection treatment in the embodiment of the present invention is a treatment to introduce cracking or strain energy to the surface of the black-cored malleable cast iron member, and is not a treatment having a strong breaking force such as removal of an oxide film on the surface of the member subjected to the plating treatment as in the prior art. When energy is applied to the surface of the member to a high degree that can remove an oxide film made of silicon oxide or the like, the formation rate of the plating layer becomes higher than necessary when forming the plating layer described later, and it is difficult to control the plating thickness, which is not preferable. Therefore, the particle projection treatment was performed so that silicon oxide remained on the surface of the black-heart malleable cast iron. The surface of the black-cored forged cast iron member may be not entirely treated with the particles, or may be a part of the surface of the member.

The particle projection process is a process of projecting particles onto the surface of the black-cored malleable cast iron member at a high speed, and is classified into a mechanical type and an air type. As the mechanical system, there is a method of projecting projection particles (medium) onto a member (workpiece) as a workpiece by using centrifugal force of an impeller (impeller). Specific examples of the mechanical treatment include shot peening such as shot blasting and shot peening, and sand peening such as sand blasting. The air type includes a method of projecting the projection particles by compressed air (air jet cleaning). In order to perform the homogeneous particle projection processing, the number of projection positions by the impeller or the compressed air is preferably 2 or more. The object to be processed (workpiece) may be agitated, rotated, or the like during processing, or may be fixed.

The material, particle diameter, hardness, and the like of the projection particles (medium) are not considered. Examples of the material of the projection particles include steel, cast steel, stainless steel, alumina, ceramics, glass, silica sand, and the like. The projection particles are preferably steel balls, steel shot, sand (sand) in this order. Examples of the shape of the projection particle include a spherical shape, a short cylindrical cut line obtained by simply cutting a metal wire, and a mesh having an acute-angled corner. The preferred range of the particle diameter of the projection particles varies depending on the size and shape of the object to be treated, the material of the projection particles, and the like. For example, when steel balls are used as the projection particles, the preferable range of the particle diameter is 5 to 10 mm. By using the projection particles having a particle diameter of 5mm or more, sufficient impact can be applied to the surface of the object to be treated. Further, by using projection particles having a particle diameter of 10mm or less, it is possible to project the projection particles onto a concave portion of a workpiece having a complicated shape, and it is possible to prevent an excessive film thickness of a plating layer due to an excessive impact. A more preferable range of the particle diameter of the steel ball used for projecting the particles is 6 to 8 mm.

As one specific example of the particle projection processing, for example, the following method can be mentioned: using steel balls having a diameter of 6mm to 8mm as projection particles, a large number of the projection particles were flapped against the surface of the black-cored malleable cast iron member using an impeller while stirring the black-cored malleable cast iron member.

In the embodiment of the present invention, since the surface of the black heart malleable cast iron member is subjected to the particle projection treatment so that the silicon oxide remains on the surface, the surface of the black heart malleable cast iron member after the particle projection treatment and before the flux treatment described later has the silicon oxide thereon. The purpose of the above treatment in the present invention is not to remove silicon oxide, and therefore silicon oxide remains on the surface of the black heart malleable cast iron member after the above treatment. The silicon oxide may remain in a large amount. For example, as described in examples below, the area ratio of the silicon oxide occupying the surface of the black heart malleable cast iron member after the particle projection treatment may be, for example, 50% or more, more preferably 70% or more, and still more preferably 90% or more, relative to the amount of the silicon oxide existing on the surface of the black heart malleable cast iron member before the particle projection treatment. Whether or not silicon oxide remains on the surface of the black-cored malleable cast iron member after the particle projection treatment can be confirmed by, for example, taking an image of an elemental map of silicon and oxygen on the surface or cross section of the sample, as shown in examples described later.

The surface of the black-cored malleable cast iron member after the particle projection treatment of the present invention has a processed and altered region by the particle projection treatment. That is, in the black-cored cast iron member formed by plating obtained as a final product, the cast iron surface of the black-cored malleable cast iron member has a work-altered region.

In the embodiment of the present invention, the projection time, the projection speed, the projection angle, the projection amount, and the like are not particularly limited as long as silicon oxide remains on the surface after the above treatment. The size of the workpiece (workpiece) can be set as appropriate according to the size (1/8-8 inches in the case of a pipe joint). From the viewpoint of a difference from the conventional treatment for removing silicon oxide, the projection time is, for example, 20 minutes or less, preferably 10 minutes or less, and may be, for example, 3.0 minutes or more. The embodiment of the present invention is different from patent document 2 in that the long shot blast cleaning is performed for 30 to 40 minutes instead of the acid cleaning in performing the light particle projection treatment.

As described above, the reason why the floating and non-plating are suppressed by performing the light particle projection processing is not sufficiently clarified, but the following is considered. As a causative substance of non-plating or floating, silicon oxide present on the surface of the black-cored malleable cast iron member may be considered. By performing the above particle projection treatment, the ferrite layer present on the surface of the black-cored malleable cast iron member is deformed and flattened, or cracks are generated in the silicon oxide. Thus, it is considered that stress is introduced into the ferrite layer and the silicon oxide layer on the surface of the black-cored malleable cast iron member, and the reaction with the plating solution is promoted. Further, the plating solution also easily reaches the silicon oxide present in a state of being embedded in the ferrite layer. It is considered that the above-described action easily causes the separation of the silicon oxide when immersed in the plating bath.

< solder treatment >

The method for producing a plated black-cored malleable cast iron member according to the present invention is characterized in that the black-cored malleable cast iron member is immersed in a flux for a specific time or longer as described above, in addition to graphitizing in a non-oxidizing and decarburizing atmosphere, not performing an acid washing treatment before the plating formation treatment, and performing the above-described light particle projection treatment on the black-cored malleable cast iron member.

As the flux used in the embodiment of the present invention, a known weakly acidic chloride aqueous solution suitable for the flux can be used. In general, flux forms a thin film on the surface of a member to be plated, and has the effect of improving wettability with a molten metal or preventing rust from occurring during the period before hot dip plating is performed, and as a result, the flux exerts the effect of making the thickness of a plating layer formed on the surface of the member to be plated uniform or improving adhesion of the plating layer. Therefore, in hot dip plating, the step of immersing the plated member in the flux cannot be omitted. The impregnation of the black-cored malleable cast iron member in the present invention into the flux exerts a characteristic action of removing a thin oxide layer generated by graphitization in addition to the above-described action.

In the embodiment of the present invention, the oxide layer formed on the surface of the black-cored malleable cast iron member is removed in the processes of dipping, casting, and graphitization in the flux, and thus the conventional oxide removal step by pickling can be omitted. Since the flux formed of the chloride aqueous solution can be reused, the disposal of the acid solution at the time of pickling is not required. Further, the chemical reaction between the black-cored forged cast iron member and the weakly acidic chloride aqueous solution used for the flux is gentler than the chemical reaction with the strong acid used in the conventional pickling, and the generation of gas during the treatment is also small. Therefore, according to the manufacturing method of the present invention, the load on the environment can be significantly reduced as compared with the conventional manufacturing method.

When the flux is an aqueous chloride solution, the concentration of the chloride in the aqueous chloride solution is preferably 10 mass% or more and 50 mass% or less. When the concentration is 10 mass% or more, the effect of removing the oxide layer becomes remarkable. Even if the concentration is increased more than 50 mass%, the effect of removal of the oxide layer is substantially unchanged. When the concentration is 50% by mass or less, the chloride consumed in the bath of the flux can be saved. Further, the formed flux is not too thick in film thickness and is easily dried. More preferably, the concentration of the chloride aqueous solution is 20 mass% or more and 40 mass% or less.

In a preferred embodiment of the present invention, the chloride contained in the flux is 1 or more of zinc chloride, ammonium chloride, and potassium chloride. More preferred fluxes are aqueous solutions containing zinc chloride and ammonium chloride. The ratio of the contents of zinc chloride and ammonium chloride in the flux is preferably 2 or more and 4 or less in terms of molar ratio with respect to zinc chloride 1. Among them, if the ammonium chloride is 3 in terms of a molar ratio with respect to zinc chloride 1, that is, if the ammonium chloride is 54% in terms of a mass ratio with respect to zinc chloride, drying is easy, and thus it is more preferable.

When the flux is an aqueous solution containing zinc chloride and ammonium chloride, the temperature of the flux is preferably 60 ℃ or higher and 95 ℃ or lower. When the temperature is 60 ℃ or higher, the effect of removing the oxide layer becomes remarkable. Since the flux is prevented from boiling at a temperature of 95 ℃ or lower, the black-cored malleable cast iron member can be immersed in the flux more safely, and the oxide layer can be removed more stably. When the temperature of the flux is 90 ℃ or higher, hydrolysis of ammonium chloride proceeds, the flux concentration becomes stable, and the effect of removing the oxide layer is improved, which is more preferable.

The preferred time for immersing the black-cored malleable cast iron member in the flux depends on the composition, concentration, temperature of the flux, the degree of deterioration of the flux, the size of the black-cored malleable cast iron member, the thickness of the oxide layer formed on the surface of the black-cored malleable cast iron member, and other conditions. The dipping time in the flux is 3.0 minutes or more, preferably 5.0 minutes or more and 60 minutes or less. When the immersion time is 5.0 minutes or more, the effect of removing the oxide layer becomes remarkable, and therefore, the immersion time is preferable. The removal effect of the oxide layer was substantially unchanged even when the immersion was carried out for more than 60 minutes. Therefore, when the dipping time is 60 minutes or less, excessive melting of the black-cored malleable cast iron member can be prevented, and the flux can be made durable. The dipping time in the flux is more preferably 10 minutes to 50 minutes, and still more preferably 15 minutes to 40 minutes. However, when the oxide layer formed on the surface of the black-cored malleable cast iron member is very thick, the member may be immersed in the flux for more than 60 minutes.

If the black-cored malleable cast iron member is repeatedly dipped in the flux, the flux is discolored to green. This is presumably because iron dissolves in the flux to generate iron (II) chloride (ferrous chloride). If use is continued, the solder changes color to a reddish brown color. This is presumably because iron (II) chloride is oxidized to generate iron (III) chloride (ferric chloride). If further use is continued, further oxidation takes place to form iron (III) hydroxide and precipitation takes place. Since iron (III) hydroxide becomes a cause of non-plating if it adheres to the surface of the black-cored malleable cast iron member, it is preferably removed from the flux by filtration. The iron (III) hydroxide is removed by filtration and the concentration of the flux is managed in a preferable range, whereby the flux temporarily prepared (bath) can be continuously used for a long time.

The control of the concentration of the flux can be performed by a known method such as the specific gravity and pH of the flux or the analysis of chemical components contained in the flux. For example, when an aqueous chloride solution containing ammonium chloride 3 in a molar ratio relative to zinc chloride 1 is used as the flux, the concentration of the aqueous chloride solution can be adjusted to a preferred range of 10 mass% to 50 mass% by adjusting the amount of solute dissolved so that the specific gravity measured at 90 ℃ is 1.05 to 1.30. Further, if the concentration of the chloride aqueous solution is adjusted so that the specific gravity measured at 90 ℃ is 1.10 or more and 1.20 or less, the concentration of the chloride aqueous solution can be adjusted to a more preferable range of 20 mass% or more and 40 mass% or less. When the concentration of the flux is reduced by continuing to use the flux, the solute is added so that the specific gravity of the flux falls within the above range, whereby the concentration of the flux can be controlled so as not to deviate from the preferable range. The specific gravity of the flux can be measured, for example, using a liquid scale. The preferable pH range of the flux used in the present invention is 3.0 or more and 6.0 or less.

< Hot dip coating >

The method for producing a black-cored malleable cast iron member formed by plating according to the present invention includes a step of hot dip plating the black-cored malleable cast iron member taken out from the flux. By hot dip coating, a coating layer can be formed on the surface of the black heart malleable cast iron member. According to the production method of the present invention, graphite is hardly generated on the surface before the plating layer is formed after graphitization, and the plating layer having excellent adhesion can be formed on the surface by the particle projection treatment and the flux treatment. As the plating layer in the present invention, a plating layer of a metal or an alloy can be used. Specifically, a metal such as zinc, tin, aluminum, or an alloy thereof may be used, but the plating layer is not limited thereto. Preferably hot dip galvanising.

In a preferred embodiment of the present invention, the step of performing hot dip plating includes a step of performing hot dip galvanizing. Zinc is preferred because it has a large ionization tendency and a sacrificial corrosion-preventing effect. When the plating first performed is hot dip galvanizing, a zinc layer (η layer) is formed on the outermost surface of the black-cored malleable cast iron member formed by plating, and an alloy layer (δ 1 layer and ζ layer) of iron and zinc is formed between the zinc layer and the surface of the black-cored malleable cast iron member. The layers are firmly adhered to each other, and a plating layer having good adhesion is formed as a whole.

In the embodiment of the present invention, by performing graphitization in a decarburizing atmosphere, as described above, a ferrite layer can be formed on the surface of the black heart malleable cast iron member before the plating layer is formed after graphitization. In this case, the ferrite layer is formed, and in this case, the ferrite reacts with the zinc to form an alloy layer. After the hot dip coating (for example, after the hot dip zinc coating is formed), the ferrite layer may remain in the coating layer or may disappear.

When the step of performing hot dip galvanizing includes the step of performing hot dip galvanizing, the temperature of the galvanizing bath used for hot dip galvanizing is preferably 450 ℃ or higher and 550 ℃ or lower. When the temperature is 450 ℃ or higher, the solidification of zinc in the galvanizing bath can be prevented. At 550 ℃ or lower, excessive reaction of the zinc-plated layer with the surface of the black-cored malleable cast iron member can be prevented. The more preferable temperature of the galvanizing bath is 480 ℃ to 520 ℃.

In a preferred embodiment of the present invention, when the step of performing hot dip plating includes the step of performing hot dip galvanizing, the galvanizing bath used for hot dip galvanizing may contain aluminum. When aluminum is melted in the plating bath, the formation of a zinc oxide film on the surface of the plating bath is suppressed, and the liquid surface is cleaned. In addition, the formed plating layer also increases the luster and improves the aesthetic feeling.

According to the method for producing a black-cored malleable cast iron member formed by plating according to the present invention, no unplating occurs even if pickling is omitted, and a plating layer can be formed by hot dip plating. The reason is not clear, but is presumed to be the following reason. The reason for 1 is that the surface of the black heart malleable cast iron member before hot dip coating after graphitization is less likely to cause no coating. Since graphitization is performed in a decarburizing atmosphere, graphite, which is one of the causative substances of non-plating, is hardly formed. Further, since graphitization is performed in a non-oxidizing atmosphere, there is almost no oxide layer and it is very thin if any.

Even if the oxide layer remains, most of the oxide layer is considered to be removed when the oxide layer is immersed in the flux after the particle projection treatment. When the dipping time in the flux is short, the gas generated during the hot dip plating adheres to the surface of the plated member in the form of bubbles, and the above-described floating may be observed. The reason for this is not clear in detail, but presumably is that if the dipping time in the flux is insufficient, substances causing gas generation remain on the surface of the black-cored malleable cast iron member. However, in the embodiment of the present invention, if the dipping time in the flux is sufficiently prolonged, the floating hardly occurs.

The 2 nd reason is that the oxide layer thinly formed on the surface of the black heart malleable cast iron member is peeled off from the surface of the black heart malleable cast iron member during the hot dip plating process and is made harmless. When the flux is an aqueous solution containing zinc chloride and ammonium chloride, iron oxide on the surface of the black-cored malleable cast iron member may chemically react with the ammonium chloride to form a black product. The product is generally difficult to peel off and becomes one of the causes of non-plating. However, in the embodiment of the present invention, a phenomenon was observed in which black products peeled off from the surface of the black heart malleable cast iron member and floated on the surface of the plating bath during hot dip plating. It is thus presumed that, in the embodiment of the present invention, even when the above-described black product is produced, since the product is peeled off during hot dip plating, no unplating occurs even if pickling is omitted.

< Heat treatment >

The black-cored malleable cast iron member formed by plating according to the present invention can be produced by dipping the black-cored malleable cast iron member in a flux and then performing hot dip plating without performing a heating treatment. As one possible embodiment of the present invention, as described in the following description, the following steps may be provided: the black heart malleable cast iron member after being taken out from the flux and before being subjected to hot dip plating is heated.

By heating the black heart malleable cast iron member before hot dip coating in advance, there is a tendency that occurrence of no coating can be suppressed. The heating temperature of the black heart malleable cast iron member depends on the size and shape of the black heart malleable cast iron member. The heating temperature when heating the black-cored malleable cast iron member is preferably 90 ℃ or higher. More preferably 100 ℃ or higher and 250 ℃ or lower. When the temperature is 100 ℃ or higher, the flux can be sufficiently dried, and the reaction of the flux with the oxide layer on the surface of the black-cored malleable cast iron member can be promoted to be harmless. At 250 ℃ or lower, the flux is not decomposed by the temperature rise, and the peeling of the flux and the additional oxidation of the surface of the black-cored malleable cast iron member can be prevented. The heating temperature is more preferably 150 ℃ to 200 ℃.

When heating is performed, a known heating means such as a heat treatment furnace may be used. For example, the black heart malleable cast iron member taken out from the flux may be inserted into a heat treatment furnace previously heated to a specific temperature, taken out from the heat treatment furnace if the temperature of the black heart malleable cast iron member reaches a preferred temperature, and hot dip plating may be performed before the temperature of the black heart malleable cast iron member is greatly lowered. In the above case, the temperature of the black heart malleable cast iron member does not need to be uniformly heated over the entire black heart malleable cast iron member, and at least the temperature of the portion of the surface of the film on which the flux is formed may be a predetermined temperature. However, when a portion of the surface to be hot-dip plated does not reach a predetermined temperature, no plating may occur on the surface of the portion. Therefore, the temperature of the entire surface to be hot-dip plated is preferably within the above-described preferred temperature range.

The time required for heating depends on the size and shape of the black heart malleable cast iron parts. For example, when hot dip coating is performed on a large-sized black-cored malleable cast iron member, it is more preferable to heat the member for a sufficient time until the temperature of the central portion of the member reaches a preferred temperature range. This prevents the surface of the black-cored malleable cast iron member from being lowered in temperature during hot dip coating, thereby preventing occurrence of unplating.

The heating may be performed in order to sufficiently suppress the occurrence of the non-plating, and as described in detail below, the iron oxide on the surface of the black-cored malleable cast iron may be sufficiently peeled off by changing the iron oxide into a black product. On the other hand, bubbles are easily generated on the surface of the black-cored malleable cast iron member by the heating. If the bubbles are generated, the floating is easily generated according to the shape of the black-cored malleable cast iron member. Therefore, depending on the shape of the black-cored malleable cast iron member, the absence of the heat treatment is also one of the embodiments of the present invention from the viewpoint of sufficiently suppressing the floating.

The phenomenon described above, in which black products are peeled off from the surface of the black heart malleable cast iron member during hot dip plating and float on the surface of the plating bath, tends to be particularly conspicuously observed when the step of heating the black heart malleable cast iron member is further included before hot dip plating is performed after removal from the flux. The reason for this is not clear in detail, and it is estimated that it is probably related to the case where the surface temperature of the black heart malleable cast iron member immediately after the black heart malleable cast iron member heated to the preferable temperature range is immersed in the hot dip plating bath is higher than the case where the member is immersed without heating. That is, in the case of immersion without heating, when the flux on the surface of the black-cored malleable cast iron member is decomposed by contact with the molten metal, the reaction temperature of the decomposition product of the flux and the iron oxide on the surface of the black-cored malleable cast iron is low, and therefore the reaction rate is slow. Therefore, it is considered that the entire iron oxide cannot be changed to a black product, and peeling is unlikely to occur. On the other hand, it is considered that when the steel sheet is immersed in a hot-dip plating bath after being heated, the reaction temperature is high, the reaction rate is also high, the reaction between the decomposition product of the flux and the iron oxide is completed in a short time, and the iron oxide is changed into a black product as a whole and can be easily peeled off from the surface of the black-cored wrought cast iron member.

The black-cored malleable cast iron member according to the present invention, which is formed by plating, contains silicon oxide in the hot-dip galvanized layer. It is assumed that the silicon oxide may be a substance which is separated from the surface of the black-cored malleable cast iron and mixed into the hot-dip galvanized layer in the process of hot-dip galvanizing, as described in detail in the following examples. Silicon oxide contained in the hot-dip galvanized layer is removed from the surface of the black-cored malleable cast iron member, and therefore, does not cause plating failure. In addition, since the black-cored malleable cast iron member formed by plating according to the present invention is subjected to a particle projection treatment in the manufacturing process, the cast iron surface of the black-cored malleable cast iron member has a work-altered region.

< pipe joint and method for manufacturing the same >

The black-cored malleable cast iron member formed by plating according to the present invention includes a pipe joint. That is, the present invention also includes a method of manufacturing a black heart malleable cast iron member formed by plating, wherein the black heart malleable cast iron member formed by plating is a pipe joint. The black-cored malleable cast iron member according to the present invention is excellent in the adhesion of the plating layer formed on the surface, and can be preferably used for a pipe joint requiring high corrosion resistance. In the case of using the plated black-cored malleable cast iron according to the present invention as a pipe joint, after hot dip plating is performed, male or female threads for connection of the joint may be provided at an end portion of the pipe joint by machining.

The black-cored malleable cast iron member and pipe joint formed by plating according to the present invention may be formed by forming a hot-dip galvanized layer, and may be formed by applying other layers such as coating with a thermosetting resin, lining with a thermosetting resin, chemical conversion treatment, sputtering of metal, thermal spraying, and the like to the hot-dip galvanized layer.

[ examples ] A method for producing a compound

[ example 1]

A molten metal containing 3.1 mass% of carbon, 1.5 mass% of silicon, 0.4 mass% of manganese, and the balance of iron and inevitable impurities was prepared. Next, 700kg of the prepared molten metal was poured into a ladle, 210g (0.030 mass%) of bismuth was added thereto, and the mixture was stirred and immediately poured into a mold, thereby casting a plurality of elbow-shaped pipe joints having dimensions shown in table 3. The content of bismuth having a high vapor pressure in the pipe joint is 0.020% by mass or less. After the cast pipe joint is taken out from the mold, shot blasting is slightly performed to remove the molding sand adhering to the surface. The maximum wall thickness of the resulting pipe joint was about 8mm and the mass per 1 piece was about 900 g.

Subsequently, the obtained pipe joint is preheated at a temperature of 275 ℃ to 425 ℃ in an atmospheric atmosphere, and then graphitized. Graphitization is performed by a 2 stage heat treatment of 1 st graphitization where the casting is held at 980 ℃ for 90 minutes and 2 nd graphitization where a temperature reduction takes 90 minutes from 760 ℃ to 720 ℃. The temperature decrease time at the time of shifting from the 1 st graphitization termination temperature to the 2 nd graphitization initiation temperature was 90 minutes.

Graphitization is performed using a heat treatment furnace in which the atmosphere in the furnace is controlled. The heat treatment furnace is supplied with the reformed gas generated by the heat generation-type reformed gas generation device. The reformed gas was produced by mixing air with a combustion gas obtained by mixing 30 vol% of propane gas and 70 vol% of butane gas and combusting the mixture. The air mixing ratio in the mixed gas of the combustion gas and the air is 95.4 vol% to 95.6 vol%.

Passing the generated reformed gas through a freeze-dehydration machine with a temperature of 2 deg.C to remove part of the water vapor, and supplying to a hot placeAnd (6) arranging the furnace. The total pressure of the converted gas supplied into the heat treatment furnace is atmospheric pressure. Sampling gas in the 1 st and 2 nd graphitization heat treatment furnaces from a sampling port, and using an infrared light absorption type CO concentration meter and CO₂The concentration of the gas was measured by a concentration meter, and the dew point of the gas was measured by a dew point meter. The obtained CO and CO in the heat treatment furnace₂The volume percentage of (A), the dew point and the estimated value of the furnace oxygen partial pressure obtained by the equilibrium calculation are shown in Table 2. The dew point corresponds to the amount of moisture contained in the gas. The balance of the gases not shown in table 2 is hydrogen and nitrogen.

[ Table 2]

The estimated value of the furnace oxygen partial pressure shown in Table 2 was compared with the equilibrium oxygen partial pressure shown in Table 1, and the furnace oxygen partial pressure for the 1 st graphitization was 3.4X 10, which is the equilibrium oxygen partial pressure of chemical formula 1^-16atm is the same as 10 to the minus 16 th power, and is an equilibrium oxygen partial pressure of chemical formula 2, i.e., 2.6X 10^-19a value of several thousand times atm. From this, it is presumed that the atmosphere for the 1 st graphitization is non-oxidizing and has strong decarburization.

Then, when observing the 2 nd graphitization, the furnace oxygen partial pressure for the 2 nd graphitization was 5.1X 10 which is the equilibrium oxygen partial pressure of chemical formula 1^-2110 times or less atm, and 2.8X 10, which is the equilibrium oxygen partial pressure of chemical formula 2^-21atm is a high value of 10 to the minus 20 th power. From this, it is estimated that the atmosphere for the 2 nd graphitization is non-oxidizing and decarburizing.

The color of the surface of the graphitized tube joint was bright gray. In the interior far from the surface, a typical structure of black-cored malleable cast iron is produced, which is composed of a ferrite matrix and massive graphite (graphite) contained in the matrix. A ferrite layer having a thickness of about 200 μm, which is composed of only a ferrite phase, is formed near the surface of the pipe joint. A thin oxide layer having a thickness of about 20 μm is formed on the outermost surface of the ferrite layer.

In comparative examples, in experiment nos. 2 to 5 in table 3, pickling was performed by immersing in an acidic solution of 10% hydrochloric acid for the time shown in table 3, before immersing in a flux bath after the graphitization.

In experiment nos. 6 and 7 of table 3, after the above graphitization, as the particle projection treatment, shot blast cleaning was performed under the following conditions. On the other hand, in experiment nos. 1 to 5 of table 3, the shot blast cleaning was not performed, and the flux impregnation described later was performed. Shot blasting was performed under the following conditions. That is, a conveyor type blasting device is used to throw a workpiece (object to be processed) into a recess of a rubber endless conveyor, and a steel ball having a diameter of about 6mm is projected onto the workpiece from a projection unit (a rotating impeller) provided at an upper portion 2 of the conveyor while changing the direction of the workpiece by rotating the conveyor. In one treatment, the amount of work put in was 400kg, and the treatment time was 10 minutes.

Next, a flux raw material containing 46 mass% of zinc chloride and 54 mass% of ammonium chloride was dissolved in tap water, the concentration was adjusted so that the specific gravity at 50 ℃ was 1.25, and then a flux bath heated to 90 ℃ was prepared. Then, the pipe joint was immersed in the solder bath for the time shown in table 3. The pipe joint taken out of the flux was inserted into a furnace chamber of a muffle furnace heated to 300 ℃ in an atmospheric atmosphere, and heated for 5 minutes. It is estimated that the surface of the pipe joint at this time is heated to 150 ℃ or higher and 200 ℃ or lower.

Then, the pipe joint was taken out of the muffle furnace, immediately immersed in a hot dip galvanizing bath, taken out after 1 minute, washed with water, dried, and cooled to produce a pipe joint of black-cored malleable cast iron having a plating layer on the surface. The hot dip galvanizing bath used had a composition of 0.03 mass% of Al and the balance of Zn. The temperature of the hot dip galvanizing bath is 500 ℃ to 520 ℃. Then, the presence or absence of floating in the plating bath was examined. The results are also shown in Table 3.

[ Table 3]

As shown in Table 3, in experiment No.1 in which pickling and shot blasting were not performed, floating occurred in the plating bath. In addition, in experiment Nos. 2 to 5 in which the acid cleaning was performed but the shot blast cleaning was not performed, the floating in the plating bath occurred. In addition, in experiments nos. 2 to 4, although comparative examples were prepared by changing the pickling time using pipe joints of the same shape, the floating was generated even if the pickling time was extended. In the pipe joint, floating occurs, and as a result, non-plating tends to occur.

In contrast, in experiment nos. 6 and 7, after shot blasting under the above conditions, flux treatment and immersion in the plating bath were performed, and therefore, no floating occurred in the plating bath. As a result, no plating was generated.

(example 2)

In both experiment No.1 (no shot blast cleaning) and experiment No.6 (shot blast cleaning) in table 3, the number of unplated occurrences was investigated by changing the immersion time in the hot dip galvanizing bath as shown in table 4 using elbow-shaped pipe joints having a nominal diameter of 3/4 inches. The same conditions as in example 1 were used except for the immersion time in the plating bath.

The appearance of the plating layer of the pipe joint obtained was visually observed to determine the presence or absence of so-called "no plating" in which no zinc plating layer was formed. 3 pipe joints were prepared for each condition, and the number of pipe joints having no plating occurred in the 3 pipe joints was determined. The results are shown in Table 4. In any of the examples shown in table 4, the thickness of the plating at 10 places where the plating was formed was measured, and the result was 70 μm or more.

[ Table 4]

As can be seen from table 4, no plating occurred when mild shot blasting was performed, but no plating occurred when shot blasting was not performed even if the immersion time in the plating bath was extended. Although not shown in table 4, it is understood that the immersion time for achieving the target film thickness (for example, 70 μm) can be shortened in the case where the shot blast cleaning is performed as compared with the case where the shot blast cleaning is not performed. As a reason for the difference, it is considered that the surface is activated by the shot blast treatment, and therefore, the plating layer is more easily formed than a case where the shot blast treatment is not performed for the same immersion time.

As is clear from the results of the present example, according to the method for producing a black-cored malleable cast iron member formed by plating according to the present invention, even if pickling after graphitization, which has been conventionally required, is omitted, a good plating layer that suppresses non-plating, preferably no non-plating, can be formed. This is considered to be because the black-cored malleable cast iron member having a surface suitable for forming the plating layer can be obtained by performing graphitization, which is indispensable for the production of the black-cored malleable cast iron member formed by plating, in a specific atmosphere, and performing light particle projection treatment, and then performing immersion in a flux under specific immersion conditions, and the plating layer can be formed well when the plating layer is formed while sufficiently suppressing floating at the time of immersion in the plating bath.

(example 3)

For the purpose of examining the action of the particle projection treatment in the present invention, the metal structure in the vicinity of the surface of the sample after the execution of each step was observed using a scanning electron microscope.

After graphitization

Fig. 1A is an example of a reflection electron image of a cross section near the surface of a black heart malleable cast iron member before the particle projection treatment after the graphitization was performed under the same conditions as in example 1. The light gray phase shown in fig. 1A is a matrix of ferrite. No bulk graphite was observed in the matrix. This is considered to be because the graphitization proceeds in a decarburizing atmosphere, and thus the chemical reaction shown in the above chemical formula 2 proceeds on the surface of the sample, and the graphite disappears. The ferrite layer formed of the ferrite matrix was present in a thickness of about 200 μm in the depth direction from the vicinity of the surface as shown in fig. 1A.

In the region from the outermost surface of the sample to a depth of about 10 μm shown in fig. 1A, a phase of a nearly spherical shape shown in dark gray in a matrix of ferrite is distributed. The size of the near spherical shaped phase is greater than about 1 μm. In a deeper region of about 10 μm from the depth of the outermost surface of the sample, an elongated phase shown in dark gray in a matrix of ferrite and a finely dispersed phase between the elongated phases were observed. The width of the elongated phase is smaller than 1 μm, and the size of the finely dispersed phase is smaller than that. The thickness of the region where the above-mentioned phase exists is about 20 μm.

Fig. 1B is an elemental mapping image of silicon in the same area as fig. 1A. Fig. 1C is an elemental mapping image of oxygen in the same region as fig. 1A. The position of the silicon and oxygen distribution is in good agreement with the position of the above-mentioned dark grey phase distribution in fig. 1A. In addition, according to the element mapping image of iron not shown in the same region as fig. 1A, iron is deficient in the silicon and oxygen-enriched portion. From the above facts, it is considered that the dark gray phase is not a phase of iron oxide but a phase of silicon oxide. Silicon is an element contained in black heart malleable cast iron parts. The elongated phase is considered to be a silicon oxide phase formed along the grain boundary of ferrite. In addition, it is considered that the finely distributed phase is a silicon oxide phase formed in the crystal grains of ferrite.

Fig. 2 is an example of a reflection electron image of the surface of a black heart malleable cast iron member before the particle projection treatment after the graphitization was performed under the same conditions as in example 1. From an unillustrated element map image in which the same area as fig. 2 is captured, it is considered that the light gray or dark gray area in fig. 2 is silicon oxide, and the white area and the white particles are ferrite composed of iron, which is a heavy element.

After particle projection processing (shot blast cleaning)

Fig. 3 is an example of a reflection electron image of a cross section near the surface of a black heart malleable cast iron member after shot blasting performed as a particle projection process under the same conditions as in example 1 until solder dipping. As in fig. 1A, the light gray phase is a ferrite matrix, and the dark gray phase is a silicon oxide phase. A flat texture was observed on the surface of the sample, and a void was observed thereunder. In addition, no phase of silicon oxide having a shape close to the spherical shape observed in fig. 1A was found. In addition, the thickness of the region where the silicon oxide phase is distributed is thinner than that of fig. 1A. In addition, it should be noted that the magnification is different in the above-described fig. 1A and 3.

Fig. 4 is an example of a reflection electron image of the surface of a black heart malleable cast iron member before flux dipping after shot blasting as a particle projection process under the same conditions as in example 1. The white particles of ferrite observed in fig. 2 are hardly observed in fig. 4, and instead, flat ferrite indicated by a white area is observed. Further, it was confirmed that cracks were generated in the ferrite, and phases of silicon oxide shown in dark gray in the cracked portions of the ferrite were distributed in large numbers in the form of grains. In addition, granular silicon oxide is also present on the surface of the flat portion of ferrite.

From the observation of the photographs of fig. 1 to 4, it is considered that the region of the phase containing ferrite and silicon oxide having a nearly spherical shape, which is distributed on the outermost surface of the black heart malleable cast iron member before the particle projection treatment after graphitization, is partially removed by shot blasting or is crushed in the depth direction to be plastically deformed. That is, the region close to the surface shown in fig. 3 is an example of the "work-altered region" in the present invention.

Further, it was confirmed from the observation of the photographs of fig. 1 to 4 that the phases of the silicon oxide were not completely removed by shot blasting, but remained in large amounts. In particular, it was confirmed that the phase of the silicon oxide finely dispersed between the elongated phases observed in fig. 1A also remained largely at a deep position far from the surface in fig. 3. That is, in the embodiment of the present invention, as shown in the above-described photograph, the particle projection treatment is performed so that silicon oxide remains on the surface. In contrast, in the conventional technique, when the oxide phase or the work-modified region is removed by pickling, all the silicon oxide is also removed. Therefore, the photographs shown in fig. 3 and 4, in which silicon oxide is present on the surface of the black heart malleable cast iron member before dipping in flux as the pretreatment for hot dip plating, can be an indirect basis for not being pickled.

After hot dip coating

Fig. 5 is an example of a reflection electron image of a cross section of the total thickness of the plating layer including the black heart malleable cast iron member after hot dip plating was performed under the same conditions as example 1, except that heating after flux dipping was not performed. The dark grey area from below to 1/4 of fig. 5 is a cross section of the black heart malleable cast iron part, and the light grey area above it is a cross section of the hot dip galvanized layer. The thickness of the hot dip galvanized layer is about 70 μm. The boundary between the 2 regions is flat without any gap. As is clear from the element mapping image of iron not shown in the same region as that of fig. 5, 1/3 regions below the plating layer contain a large amount of iron, and 2/3 regions above the plating layer contain almost no iron. Thus, it is considered that at least 2 regions, a region composed of a solid solution of iron and zinc in the vicinity of the boundary with the black-cored malleable cast iron surface and a region composed of a phase in which a slight amount of iron is dissolved in pure zinc located outside thereof, exist in the hot-dip galvanized layer.

Black phases are distributed in the region near the surface of the black-cored malleable cast iron member in fig. 5 and in the central region in the thickness direction of the hot-dip galvanized layer. From the silicon element map image and the oxygen element map in the same region as in fig. 5, which are not shown, the positions of the silicon and oxygen distributions are in close agreement with the positions of the black phase distributions in fig. 5. In addition, according to an element mapping image of zinc not shown in the same region as that of fig. 5, zinc is absent in a portion where silicon and oxygen are enriched. From the above facts, it is considered that the black phase is not a zinc oxide but a silicon oxide phase.

Fig. 6 is an example of a reflection electron image of the vicinity of the boundary between the cast iron surface and the hot-dip galvanized layer of the black-cored malleable cast iron member hot-dipped under the same conditions as in example 1, except that heating after flux dipping was not performed. The lower dark grey area of fig. 6 is a section of the black heart malleable cast iron part, and the upper light grey area is a section of the hot dip galvanized layer. The black silicon oxide phase is present in the vicinity of the surface of the black cored wrought iron that contacts the hot-dip galvanized layer, and is caused by the structure of the work-affected layer shown in fig. 3. In addition, in the hot-dip galvanized layer, a phase of silicon oxide exists at a position slightly apart from the boundary with the black-heart malleable cast iron surface in the upper part of fig. 6. On the other hand, there is almost no phase of silicon oxide at a position near the boundary with the black heart malleable cast iron surface.

The observation of the photographs of fig. 5 and 6 is estimated as follows. The flux and plating bath do not contain silicon-containing compounds, so it is assumed that the phases of silicon oxides contained in the hot dip galvanized layers in fig. 5 and 6 and the observed black phase are: silicon oxides present in the work-altered regions of the black-cored malleable cast iron member shown in fig. 3 are separated from the surface of the black-cored malleable cast iron member during the hot-dip galvanizing treatment and are mixed into the hot-dip galvanized layer.

In the hot dip galvanizing treatment, the residual stress in the processing-modified region is large and a large number of voids are contained, and therefore, the reaction with the hot dip galvanizing occurs violently. It is considered that in this reaction process, the oxide layer containing silicon oxide is separated from the surface of the black-cored malleable cast iron member and is dispersed, and as shown in fig. 5 and 6, it remains in a state of being dispersed in the hot-dip galvanized layer. Therefore, the presence of silicon oxide dispersed in the hot-dip galvanized layer can be an indirect basis for performing particle projection processing such as shot blasting so as to leave silicon oxide on the surface of the graphitized black-cored malleable cast iron member that has not been pickled.

As described above, the distribution of the phase of the silicon oxide is small in the vicinity of the boundary with the surface of the black-cored malleable cast iron. The reason for this is not clear, but it is considered that this is probably because, when a region composed of a solid solution of iron and zinc is formed in the hot-dip galvanized layer, silicon oxide is discharged into molten zinc without being mixed into the solid solution, and then, when the region containing a small amount of iron and a large amount of zinc is solidified, the discharged silicon oxide is solidified while being contained therein. It is considered that the fact that there is almost no phase of silicon oxide in the plating layer at a position close to the boundary of the surface of the black heart malleable cast iron means that no plating is prevented when the plating layer is formed.

The disclosure of the present specification includes the following embodiments described in japanese patent application No. 2019-053581 as a basis for priority claims.

Mode 1:

a method for manufacturing a black-cored malleable cast iron member formed by plating, wherein,

graphitizing the graphite body in a non-oxidizing and decarburizing atmosphere;

a step of performing particle projection treatment on the surface of the graphitized black-cored malleable cast iron member;

a step of taking out the flux and then heating the black-cored malleable cast iron member to 90 ℃ or higher;

and a step of hot dip coating the heated black heart malleable cast iron member.

Mode 2:

the method for producing a black-cored malleable cast iron member formed by plating according to mode 1, wherein the non-oxidizing and decarburizing atmosphere has an oxygen partial pressure 10 times or less a balanced oxygen partial pressure of chemical formula 1 and higher than a balanced oxygen partial pressure of chemical formula 2.

[ chemical formula 5]

2Fe(s)+O₂(g)＝2FeO(s) (1)

[ chemical formula 6]

2C(s)+O₂(g)＝2CO(g) (2)

Mode 3:

the method for producing a black-cored malleable cast iron member formed by plating according to mode 1 or 2, wherein the black-cored malleable cast iron member dipped in the flux has silicon oxide on a surface thereof.

Mode 4:

the method for manufacturing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 3, wherein the particle projection processing is any one of shot blast cleaning, shot peening, sand blast cleaning, and air blast cleaning.

Mode 5:

the method for producing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 4, wherein the particle projection treatment is performed for 3.0 minutes to 20 minutes.

Mode 6:

the method for producing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 5, further comprising a step of preheating the black heart malleable cast iron member at a temperature of 275 ℃ or higher and 425 ℃ or lower, prior to the step of graphitizing.

Mode 7:

the method for producing a black-cored malleable cast iron member formed by plating according to any one of aspects 1 to 6, wherein the step of performing graphitization includes 1 st graphitization in which heating is performed at a temperature exceeding 900 ℃, and 2 nd graphitization in which a start temperature is 720 ℃ or more and 800 ℃ or less, and an end temperature is 680 ℃ or more and 780 ℃ or less.

Mode 8:

the method of manufacturing a black-cored malleable cast iron member formed by plating according to mode 7, wherein in the graphitizing step, at least the 1 st graphitization is performed in a non-oxidizing and decarburizing atmosphere.

Mode 9:

the method for producing a black-cored malleable cast iron member formed by plating according to any one of aspects 1 to 8, wherein the non-oxidizing and decarburizing atmosphere includes a converted gas generated by combustion of a mixed gas of a combustion gas and air.

Mode 10:

the method for producing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 9, wherein, in the step of heating the black heart malleable cast iron member after taking out from the flux, the black heart malleable cast iron member is heated to 100 ℃ or higher and 250 ℃ or lower.

Mode 11:

the method for producing a black heart malleable cast iron member by plating according to any one of embodiments 1 to 10, wherein the flux is an aqueous solution containing a weakly acidic chloride.

Mode 12:

the method for producing a black heart malleable cast iron member by plating according to any one of aspects 1 to 11, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

Mode 13:

the method for producing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 12, wherein the step of performing hot dip plating includes a step of performing hot dip galvanizing.

Mode 14:

the method for manufacturing a black heart malleable cast iron member formed by plating according to any one of aspects 1 to 13, wherein the black heart malleable cast iron member is a pipe joint.

Mode 15:

a black heart malleable cast iron member formed by plating, wherein,

the coating layer is a hot dip galvanizing layer,

the hot-dip galvanized layer includes silicon oxide.

Mode 16:

the black-cored malleable cast iron member formed by plating according to mode 15, wherein the black-cored malleable cast iron member formed by plating is a pipe joint.

The present application claims priority on the basis of Japanese patent application No. 2019-053581. The contents of Japanese patent application No. 2019-053581 are incorporated in the present specification by reference.

Claims

1. A method for manufacturing a black-cored malleable cast iron member formed by plating,

the black-cored malleable cast iron member formed by plating has a plating layer formed on the surface of the black-cored malleable cast iron member, and the manufacturing method comprises the following steps:

graphitizing the graphite body in a non-oxidizing and decarburizing atmosphere;

2. The method for producing a black heart malleable cast iron part formed by plating according to claim 1, wherein the non-oxidizing and decarburizing atmosphere is an atmosphere having an oxygen partial pressure 10 times or less higher than an equilibrium oxygen partial pressure of the following chemical formula (1) and higher than an equilibrium oxygen partial pressure of the following chemical formula (2),

2Fe(s)+O₂(g)＝2FeO(s) (1)

2C(s)+O₂(g)＝2CO(g) (2)。

3. the method for manufacturing a black heart malleable cast iron part formed by plating according to claim 1 or 2, wherein the particle projection treatment is any one of shot blasting, shot peening, sand blasting, air blasting.

4. The method for manufacturing a black heart malleable cast iron member formed by plating according to any one of claims 1 to 3, wherein the particle projection treatment is performed for 3.0 minutes or more and 20 minutes or less.

5. The method for producing a black heart malleable cast iron member formed by plating according to any one of claims 1 to 4, wherein the production method further comprises a step of preheating the black heart malleable cast iron member at a temperature of 275 ℃ or higher and 425 ℃ or lower before the step of graphitizing.

6. The method for manufacturing a black heart malleable cast iron member formed by plating according to any one of claims 1 to 5, wherein the graphitizing step comprises: 1 st graphitization by heating at a temperature exceeding 900 ℃; and 2 nd graphitization with a start temperature of 720 ℃ to 800 ℃ inclusive and an end temperature of 680 ℃ to 780 ℃ inclusive.

7. The method for producing a black heart malleable cast iron part according to claim 6, wherein, in the step of performing graphitization, at least the 1 st graphitization is performed in a non-oxidizing and decarburizing atmosphere.

8. The method for manufacturing a black heart malleable cast iron member formed by plating according to any one of claims 1 to 7, wherein the non-oxidizing and decarburizing atmosphere includes a converted gas generated by combustion of a mixed gas of a combustion gas and air.

9. The method for manufacturing a black heart malleable cast iron part formed by plating according to any one of claims 1 to 8, wherein the manufacturing method further comprises: and a step of heating the black-cored malleable cast iron member to 90 ℃ or higher after the removal from the flux.

10. The method for manufacturing a black heart malleable cast iron part formed by plating according to any one of claims 1 to 9, wherein the flux is an aqueous solution containing a weakly acidic chloride.

11. The method for manufacturing a black heart malleable cast iron part formed by plating according to any one of claims 1 to 10, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

12. The method for producing a black heart malleable cast iron part formed by plating according to any one of claims 1 to 11, wherein the step of performing hot dip plating includes a step of performing hot dip galvanizing.

13. The method for manufacturing a black heart malleable cast iron part formed by plating according to any one of claims 1 to 12, wherein the black heart malleable cast iron part is a pipe joint.

14. A black-cored malleable cast iron member formed by plating,

the black-cored malleable cast iron member formed by plating is provided with a plating layer on the surface of the black-cored malleable cast iron member,

the coating layer is a hot dip galvanizing layer,

the cast iron surface of the black-cored malleable cast iron member has a work-deteriorated region, and

the hot dip galvanized layer includes silicon oxide.

15. The plated, formed black heart malleable cast iron component of claim 14, wherein the plated, formed black heart malleable cast iron component is a pipe joint.